Digital video broadcasting (DVB) employs compression protocols such as MPEG. MPEG produces data packets at variable rates, because the rate of data compression is not constant throughout a broadcast but rather depends on the rate of change of details within the frames being sent. In order to multiplex such data efficiently onto a single broadcast channel it is usual to use statistical multiplexing techniques. This allows for the introduction of variable factors affecting the timing of data packet transmission from a sending device to receiving devices.
In order to provide interactive television, or data applications over digital TV channels or satellite, for example Internet browsing, it is necessary for the receiving device to send data packets back to the sending device. Common bi-directional communication protocols, however, require that the receiving device reconstruct a timing signal generated by the sending device and transmit only in synchronization with the timing signal. For example, the Slotted ALOHA protocol, developed at the University of Hawaii, allows data packets to be sent by the receiving device only at the beginning of a time slot defined by the sending device, so that contention between two receiving devices transmitting simultaneously can be resolved with minimal data loss.
In the case of statistically-multiplexed transmissions, however, the receiving unit cannot rely on the times of receipt of the data packets to reconstruct the sending clock, because the times of receipt include the variable delays, or jitters, that were introduced by the multiplexing stage.